Metal matrix composites have increasingly been used as replacement materials for conventional metal alloys because they possess many significant advantages. Among the advantages are superior strength-to-density and/or modulus-to-density ratios, improved fatigue and fracture toughness properties, and higher thermal conductivity and lower thermal expansion in the fiber direction. These properties contribute to improved rigidity and dimensional stability.
Metal-matrix composites often must be fastened to other products to form end products, and their fastening to themselves or to conventional structural alloys is problematical. Mechanical fastening, diffusion bonding, laser welding, electron beam welding, brazing and various fusion welding techniques all present problems.
Mechanical fastening produces stress concentrations in the vicinity of fastener holes resulting in a reduction of mechanical properties. In addition, such joints are potentially susceptible to crevice corrosion. Although brazing is suitable for lap joints, the extended exposure to the high temperatures required for brazing may cause the matrix metal to react with the reinforcing fibers, leading to loss of filament strength. Although in fusion welding, the matrix metals usually melt without harming the fiber materials because the melting point of most matrix materials is appreciably lower than those of the fibers, fiber damage during fusion welding depends upon the reactivity of the fibers and the duration of contact between the fiber and the molten metal. In the case of boron/aluminum composites, arc welding causes severe fiber damage, poor metal flow, and lack of wetting of the fibers by the molten aluminum matrix. The addition of filler material eliminates some of these difficulties.